Nonpigmentary titanium dioxide is used in the manufacture of various glass and ceramic products for a wide variety of purposes. For example, it is employed in the manufacture of glass frit used in producing vitreous enamel and glaze type coatings and in the manufacture of glass ceramics employed in missile radomes and microwave transmitters. In the former application, the titanium dioxide functions as both an opacifying agent and to enhance the acid resistance of the coatings while in the latter application, it functions as a nucleating agent to effect the nucleation and growth of the major crystalline phases of the glass. Other properties of various glass and ceramic compositions which can be effected through the use of titanium dioxide are those of thermal expansion, chemical durability, refractive index, and the like.
In general, it is believed that the commercially available nonpigmentary titanium dioxides currently employed in the glass and ceramic industries are those produced by the well-known sulfate process. This belief is premised on the fact that nonpigmentary titanium dioxide products readily can be produced through the manipulation of either the crystallization step or calcination step, or both, employed in the sulfate process. Such belief is supported by the disclosures found in U.S. Pat. No. 3,434,853 issued on Mar. 25, 1969. According to this patent, a nonpigmentary titanium dioxide granular material can be produced by calcining, at a temperature of from 800.degree. C. to 1000.degree. C., a titanium hydrate prepared by hydrolyzing a titanium sulfate solution of low titanium concentration at low temperatures to form nodules of aggregated titanium dioxide particles and thereafter, subdividing these nodules into granules. The granules are described as being free flowing and capable of undergoing rapid melting and thorough dispersion when incorporated into a molten glass batch composition.
In contrast, pigmentary titanium dioxide produced by the chloride process, the second major commercial process employed for the manufacture of titanium dioxide, often is unsuited for use in glass and ceramic manufacture. Typically, pigmentary titanium dioxide is so fine and of such low bulk density that it tends to float upon the surface of the glass melt and, as a result, is easily lost through dusting as it is carried out of the glass making vessel by the hot air convection currents generated in the vessel. Fine particle size pigmentary titanium dioxide also tends to form agglomerates in the molten glass batch which agglomerates do not melt properly and which sink to the bottom of the vessel where they form a sintered mass in the glass melt.